1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device capable of optimally selecting an activated word line address when a refresh command is input.
Priority is claimed on Japanese Patent Application No. 2009-217405, filed Sep. 18, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a dynamic random access memory (DRAM) that stores data by accumulating charges in a capacitor, the charges accumulated in the capacitor leak over time, such that data cannot be detected. Accordingly, it is necessary to perform a refresh operation periodically. This refresh operation is performed by activating word lines of the DRAM, reading data from memory cells connected to the activated word lines to bit lines, amplifying a voltage difference between a pair of bit lines using a sense amplifier, and then rewriting data to the memory cells via the bit lines.
In a dynamic random access memory (DRAM) that stores data by accumulating charges in a capacitor, the charges accumulated in the capacitor leak over time, such that data cannot be detected. Accordingly, it is necessary to perform a refresh operation periodically. This refresh operation is performed by activating word lines of the DRAM, reading data from memory cells connected to the activated word lines to bit lines, amplifying a voltage difference between a pair of bit lines using a sense amplifier, and then rewriting data to the memory cells via the bit lines.
For example, in the refresh operation of a 1-gigabit (Gbit) DRAM, when an external refresh command is input once, 16k memory cells in each of eight banks are refreshed. Here, the bank refers to one unit for an independent operation, such as a read operation or a write operation. The bank includes a plurality of memory mats each having a plurality of memory cells provided at intersections between a plurality of word lines and a plurality of bit lines. Each bank performs an operation according to an input command, but in a refresh operation, eight banks are simultaneously selected and a total of 128k memory cells are refreshed.